The vote — 51 against and 43 in favor — marks the third time in the past 18 months that lawmakers have rejected efforts to legalize same-sex marriage, making Northern Ireland the only part of the UK which has not legalized marriage equality.

“Politicians in Northern Ireland who continue to block marriage rights for same-sex couples are like latter-day King Canutes, trying in vain to hold back the tide of equality,” said Patrick Corrigan, Northern Ireland director of Amnesty International.

“With politicians continuing to block equality, it is now inevitable that same-sex couples in Northern Ireland will take a legal challenge on the basis of inferior treatment with regards to the right to marry and found a family,” he said.

Opposition to the measure was led by the Catholic Church, whose bishops had urged politicians to reject “marriage equality” for same-sex partnerships in Northern Ireland while the Church of Ireland restated its opposition to any change to the traditional definition of marriage.